1. Field of the Invention
The present invention generally relates to an optical information recording/reproducing system and, more particularly, to a magneto-optical reproducing apparatus having a reproducing waveform correction circuit used for high-density or fast-transfer recording, and a signal reproducing apparatus for an optical disk device, which reproduces an information signal from an optical disk in which data is recorded by mark edge recording.
2. Description of the Related Art
A mark length recording method in which information is recorded at a mark edge is conventionally known as a method for realizing high-density recording in an magneto-optical disk.
In this method, when a recording signal is to be reproduced from a magneto-optical disk, a reproduced signal is binarized at a fixed slice level to detect an edge position. At the same time, a reproducing clock determined by a modulation method is detected to determine a pattern "0" or "1" from a timing relationship with a data detection window, thereby reproducing information.
In this information reproduction, DC variations according to envelop variations of the reproduced signal may be caused due to birefringence of the substrate of the magneto-optical disk. For this reason, the reproducing apparatus of this type uses a capacitance-coupled amplifier corresponding to a high-pass filter as an amplifier to suppress the DC variation component, thereby reproducing the information.
However, in the reproduced signal output from such a capacitance-coupled amplifier corresponding to a high-pass filter, a change in DC level between the ID area of the preformat area having information of the address or track number of the data in advance and the recording data area, or a change in DC level depending on the pattern of the recording data is also suppressed. For this reason, when the reproduced signal is binarized by the fixed slice method, the data may be erroneously read.
That is to say, since the low-frequency component in the reproduced signal is lost, baseline variations are caused to generate a transient in the signal passing through the high-pass filter. This transient is prominent at a portion between the blank portion of the ID portion at the sector head and the VFO portion as the recording portion.
In mark position recording, differential detection in which the reproduced signal is binarized by peak detection is performed, so the low-frequency component is not necessary, and the transient does not pose a serious problem.
However, when data recorded by mark edge recording is to be binarized by the DC slice method, the reproduced signal cannot be accurately binarized because of the transient due to the data pattern at the sector head or in the sector.
Therefore, a method in which the envelope of the reproducing waveform is detected to compensate the reproducing waveform is conventionally used as a means for restoring the DC component necessary for data reproduction, which is lost by capacitive coupling, and correcting the reproducing waveform.
Jpn. Pat. Appln. KOKAI Publication No. 61-39236 discloses a correction circuit for correcting a reproducing waveform. FIG. 14 is a block diagram showing the basic arrangement of this correction circuit. A reproduced signal is directly supplied to an arithmetic circuit 64 through a coupling capacitor 61 corresponding to a high-pass filter. At the same time, a result obtained when half-wave rectification of the reproduced signal is performed by a half-wave rectifier 62, and thereafter, envelope detection of the reproduced signal is performed by an envelope detector 63. The result is supplied to the arithmetic circuit 64.
When a reproduced signal having a low-frequency swell as shown in FIG. 15A is supplied to the correction circuit with the above arrangement, the low-frequency swell is removed through the coupling capacitor 61 corresponding to a high-pass filter, resulting in a waveform as shown in FIG. 15B. In this case, the reproduced signal passing through the coupling capacitor 61 has a waveform from which a DC component necessary for reproduction is lost.
The waveform shown in FIG. 15B, which is obtained through the coupling capacitor 61, is rectified by the half-wave rectifier 62, as shown in FIG. 15C. When time integration of this rectified waveform is performed by a low-pass filter of the envelope detector 63, a DC component as shown in FIG. 15D is reproduced by envelope detection of the base side of the waveform shown in FIG. 15C.
The output shown in FIG. 15D, which is obtained upon envelope detection by the envelope detector 63, and the output shown in FIG. 15B, which is obtained through the coupling capacitor 61, are sent to the arithmetic circuit 64. When the two outputs are added in opposite polarities, a waveform shown in FIG. 15E, which has the corrected DC component, can be obtained.
According to the correction circuit using envelope detection, however, there is a time delay due to the integration time constant for time integration of the rectified waveform through the low-pass filter of the envelope detector 63, in addition to a response delay of the diode in rectification by the half-wave rectifier 62. For this reason, correction of the reproducing waveform by the arithmetic circuit 64 may be delayed.
Such a time delay poses no problem when the reproduced signal has a low frequency. However, in high-density or fast-transfer recording at a high reproducing/recording frequency, this time delay largely affects the correction waveform. As shown in FIG. 15F, the corrected waveform has a large sag at the data start portion immediately subsequent to the blank portion. For this reason, the waveform may be insufficiently corrected while containing an envelope distortion due to the time delay even in the middle of data.
For this reason, this method cannot be applied to a device such as a recent magneto-optical disk device with a high transfer rate and a high operation speed.
Conventionally, to minimize the time delay, a diode with a high response speed is used, or the filter time constant is decreased. Even with a high-speed diode, however, it is difficult to obtain ideal rectifying performance because the parasitic capacitance of the diode influences a signal having a high frequency of several tens MHz. In addition, when the filter time constant is decreased to heighten the cutoff frequency, the DC component cannot be sufficiently extracted in envelope detection. Furthermore, when the time constant is sufficiently larger than the signal period, the envelope cannot be accurately detected.
On the other hand, as a waveform correction method without using envelope detection, a method called "quantization feedback" is introduced in chapter V of "Digital Communication Technology" pp 158-161 (Kimio Tanaka, Tokai Daigaku Shuppan). In this method, however, the signal is reproduced while including the low-frequency noise which is supposed to be removed.
A circuit called an AGC (Auto Gain Control) for the purpose of aligning the amplitude of the reproduced signal is arranged for a magneto-optical disk. Jpn. Pat. Appln. KOKAI Publication No. 4-95275 discloses a method in which the amplitude of the reproduced signal in the VFO portion at the sector head is detected by envelope detection to control the gain of the gain control amplifier, and the gain at the end of the VFO portion is held in the data portion subsequent to the VFO portion.